Conventionally, it has been proposed, for instance in Japanese utility model laid open (kokai) publication No. 61-182580, for the purpose of identifying or determining the authenticity of information storage cards (such as magnetic cards, various monetary papers, tickets and commercial goods) to affix an identification sticker including a hologram which is hard to forge, so that the authenticity may be visually determined.
Japanese patent laid open (kokai) publication No. 3-71383, filed by the same applicant, discloses a system for identifying an object by affixing an identification sticker which includes a hologram that contains a unique diffractive property, on the object to be identified, and radiating light upon this hologram so as to be diffracted by the hologram, and then receiving the diffracted light with an optical identification means.
In particular, according to the system using the optical identification means, as it is required for a potential forger to duplicate the diffractive property of the authentic hologram instead of the mere visual appearance, the forgery of the hologram is made highly difficult, making a reliable identification possible.
However, according to this system, it is clear to the user that the diffractive property of the hologram is providing the means for identification, and since it is not impossible to obtain the identification sticker itself (due to its nature) one cannot exclude the possibility of the hologram being forged in a highly accurate manner. Thus, there has been a demand for more reliable identification systems.
The inventors of the present application have conceived of the idea .(for the purpose of more positively discouraging forgery) that it is possible to use, in combination with the reflective directivity (or diffraction property), a light source for emitting linearly polarized light, a polarization plane rotating means provided in the identification element for rotating the polarization plane by 90 degrees, by shifting the phase of the polarized light by 180 degrees, and light receiving means which is sensitive only to the portion of the light emitted from the identification element whose polarization plane has been rotated by 90 degrees; thus the authenticity of the object may be identified according to the state of polarization.
According to this idea, because the potential forger must duplicate not only the hologram or diffraction grating but also the polarization plane rotating means, any attempt to forge the identification element can be more effectively discouraged. However, because the intensity of the received light would not change significantly even when the amount of phase shift substantially deviated from 180 degrees, the intensity of the received light alone would not be sufficient for accurately detecting the phase shift of 180 degrees or the authenticity of the identification element. It is indeed possible to place a polarization plane rotating layer, in the form of a sticker or the like, on the identification element for the purpose of achieving the phase shift of approximately 180 degrees for deceptive purposes. In short, according to this idea, there is a possibility that an attempt to duplicate the rotation of the polarization plane by using a crude means may succeed.
It is therefore desired to provide more effective means for making the duplication of an identification element based on diffraction or reflective directivity more difficult, and thereby more effectively discouraging any attempt to forge the identification element.